We know that the tumor represents essentially an abnormal growth, and that this growth is due to new formation of cells.

In certain cases the tumor repeats the structure of the organ or tissue in which it originates, in others it departs widely from this; always, however, its structure resembles structures found in the body at some period of life.

The tumor cells, like all other cells of the body, grow by means of the nutriment which the body supplies; they have no intrinsic sources of energy.

The great problem is what starts the cells to grow and why the growth differs from that of normal tissue, why it is not regulated and cooerdinated as are other forms of growth.

When a small piece of the skin, for instance, is cut out growth as rapid as that in tumors takes place in the adjoining cells, _but it ceases when the loss is restored_.
The same is true when a piece of the liver is removed.
Various hypotheses have been formed to explain the tumor, all of them of interest, and they have had great importance in that the attempt to prove or disprove the hypothesis by continued observation and experiment along definite lines has produced new knowledge.